The present invention relates generally to titanium orthopedic implant devices and, more particularly, to a surface hardening process applicable to such devices, wherein surface hardness and wear resistance properties of the implant are enhanced with minimal loss in fatigue strength.
Commercially pure titanium and titanium alloys are used for orthopedic applications because of their strength, corrosion resistance, and biocompatibility. However, the tribological behavior of titanium and its alloys is characterized by a high coefficient of friction and poor wear performance, resulting in a tendency for titanium and its alloys to seize or gall under conditions of wear. Therefore, in those orthopedic applications requiring enhanced wear resistance properties, the surface of a titanium implant must be hardened. In the past, surface hardening of orthopedic implants has been achieved either by depositing a nitride coating on the surface of an implant, or by forming a layer of titanium nitride (TiN) on the surface of a titanium substrate.
A TiN layer is produced on the surface of a titanium implant by various nitriding methods, including gas nitriding, chemical salt bath nitriding, plasma or ion nitriding, and ion implantation. Of these alternatives, gas nitriding is believed to be the earliest method used for hardening titanium, and still exhibits advantages over the other methods in terms of cost and ease of manufacture. For instance, gas nitriding permits efficient batch processing of many parts concurrently in a furnace chamber; whereas, the plasma nitriding and ion implantation methods require line-of-sight bombardment of the workpiece, thereby limiting the number of parts that may be processed concurrently.
Gas nitriding of titanium and its alloys has historically been performed at elevated temperatures in the range of 700.degree. C. to 1200.degree. C. (1292.degree. F. to 2192.degree. F.) U.S. Pat. No. 4,768,757 discloses a method for nitriding the surface of a titanium dental cast, wherein it is stated that the temperature generally used for the nitriding treatment falls in the range of 700.degree. C. to 880.degree. C. because nitriding generally begins to proceed in the neighborhood of 700.degree. C. and the heat distortion or phase transition point of titanium is about 882.degree. C. Characteristic of virtually all gas nitriding processes is the formation of a relatively thick TiN layer on the surface of the titanium, caused by a scaling reaction. Essentially, successful nitriding of a titanium orthopedic implant for the purpose of providing a hardened surface is defined by the observance of a distinct and measurable TiN layer achieved by elevated temperatures, as taught by the prior art.
It has now been discovered that the aforementioned gas nitriding process, as applied to a titanium orthopedic implant device, may produce several undesirable changes in the physical and mechanical properties of the device. Notwithstanding increases in overall surface hardness, the TiN layer formed on the surface of the device by gas nitriding at elevated temperatures tends to be brittle and exhibits increased surface roughness, both of which cause losses in the fatigue strength of the implant. Also, temperature induced changes in the dimensions of the titanium orthopedic implant device may occur.
Potential losses in the fatigue strength and increases in the surface roughness of a titanium orthopedic implant device are of particular concern in orthopedic applications involving load bearing prostheses in articulating contact with bone or polymers. For instance, under conditions of sliding or articulation of the nitrided implant against other surfaces, particularly bone and polymers, the increased surface roughness may produce wear debris that can act as an abrasive medium. Consequently, it is desirable to reduce the possibility of wear debris and its potential impact on the stability of orthopedic implants by enhancing the surface hardness of the titanium material without substantial losses in fatigue strength or wear resistance properties.